In Fig. REF  we investigate whether a correlation exists between the size of the user profile and the presence of popular books in the profile. Specifically, in Fig. REF , we depict the number of popular books in a user's profile over the size of the profile. As could be expected, there is a positive correlation since the more items in a user profile, the greater probability there are popular items in the profile. On the other hand, when plotting the average popularity of books in a user profile over the profile size in Fig. REF , we observe a negative correlation, which indicates that users having a smaller profile size tend to read books with higher average popularity. Same as [1]}, we define the popularity of a book as the ratio of users who have read that book.
